FIG. 1 schematically illustrates elements of a phase detection autofocus system. The system includes two parallel sets of photosensitive pixels Pa and Pb placed in the image plane of an optical system or lens L. The pixels of the first set Pa have their right half masked by a metal layer M, while the pixels of the second set have their left half masked. A collimating lens the size of the pixel is further mounted over each pixel.
With this configuration, given the inclination of the rays arriving at the pixels and collimating lenses, the pixels of the first set Pa essentially see the light rays passing through the left half of the lens L. The pixels of the second set Pb essentially see the rays passing through the right half of the lens L.
If the focus adjustment of the lens L is incorrect, the images captured by the two sets of pixels are spatially phase shifted. The autofocus system is designed to measure this phase shift and correct the lens adjustment to tend towards the alignment of the two images.
Physically hiding each photosensitive pixel distorts the characteristics of the pixels, so that their integration into a main image sensor could decrease image quality and require corrective processing. Thus, such masked pixels have often been provided in a separate sensor dedicated to focusing.
Rather than achieving a physical masking of the pixels, an electronic masking may be considered using dual pixels having two photosites, as described in US patent publication no. 2014/0078337. Such pixels may be integrated into the main image sensor. Then, during a focusing phase, the system uses a single photosite of the dual pixels, i.e., the left or right one depending on the row. For shooting the final image, the system uses the two photosites of each pixel and adds the measured values.
However, in a same surface area and a same manufacturing technology, a dual pixel is less efficient than a single pixel, especially in terms of Charge-Voltage conversion Factor, or CVF. In particular, the conversion factor CVF contributes to a good dynamic range of the image.